In situ leaching operations for recovering uranium from formations involve oxidizing the insoluble tetravalent uranium to its hexavalent form and solubilizing it. For example, CO.sub.2 /O.sub.2 leaching solutions, which are often used for in situ leaching operations, result in oxidation EQU UO.sub.2 +[O]+H.sub.2 O.fwdarw.UO.sub.2.sup.+2 +2 OH.sup.-
and in formation of a soluble uranyl carbonate complex, EQU UO.sub.2.sup.+2 +3 CO.sub.3.sup..dbd. .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4.
Practically, one embodiment of the CO.sub.2 /O.sub.2 leaching solution involves injecting a solution of CO.sub.2 and O.sub.2 through at least one injection well into the subterranean formation, allowing leaching to occur, and then pumping the leachate pregnant in uranium in the form of a soluble uranyl complex from the formation through a production well.
The leachate contains other mineral values in addition to the uranium. Ion exchange techniques are now commercially used to recover, concentrate, and selectively isolate uranium. Such ion exchange techniques depend on the existence of anionic complexes of uranium in solution which, under proper conditions, are selectively adsorbed from a leachate by suitable synthetic resins. As suggested above, the complex anion of uranium, produced during CO.sub.2 /O.sub.2 leaching, is the tetravalent uranyl tricarbonate anion, [UO.sub.2 (CO.sub.3).sub.3 ].sup.-4, which predominates although a divalent ion, [UO.sub.2 (CO.sub.3).sub.2.2 H.sub.2 O].sup..dbd. may exist at low carbonate concentrations. Generally, the adsorption of the tetravalent uranyl tricarbonate anion, by ion exchange, is accomplished by use of strong base anionic exchange resins. The strong base anionic exchange resins contain quaternary ammonium functional groups as the active anion constituent. Some are made by the chloromethylation of polystyrene and subsequent treatment with a tertiary amine. Pyridinum groups may be substituted in part for the amine groups in some resins developed specifically for uranium recovery. The strong base anionic resins are highly ionized, usable over a wide pH range, stable in the absence of strong reducing or oxidizing agents, insoluble in most of the common solvents and will withstand temperatures up to about 60.degree. C.
Notwithstanding the aforementioned properties of those resins, the performance of strong base anionic exchange resin deteriorates during usage, due to the repeated conditions of loading (adsorption) and elution and due to poisoning. Performance deterioration of the resin is manifested by a decrease in loading capacity and early uranium "leakage". Performance deterioration may be irreversible if due to complete and actual removal of at least some of the active ion groups, but reversible if due to constructive removal of those active ion groups which form strong physical or chemical bonds with poisons and fouling agents.